> Also, it's very hard to figure out during interviews if the person is "smarter". What does that mean? They can solve more Leetcode problems?

Not at all. "Smarter" and "spent enough time grinding leetcode" are almost entirely orthogonal. Trying to measure "smarter" by leetcode is even more misguided than trying to measure "good candidate" by leetcode.

I've interviewed a fair number of people. During the interview, we ask them questions, trying to figure out what they know. With this one candidate, though, halfway through the interview we changed - we started asking questions because we wanted to learn things.